Magnetic lines of force in ferromagnetic articles are normally regular and predictable, and this phenomenon has long been used to reveal flaws as part of an inspection process in the production of such articles as pipes. While some techniques have involved the application of an electric current directly to the article to be inspected, in order to generate magnetic lines of force, this practice has been considered dangerous and has other disadvantages, and accordingly the discussion below is restricted to those techniques and configurations wherein magnetic lines of force are induced by the use of electromagnets or other magnetic field inducing devices which need not touch the scrutinized articles. The reader may be interested, however, in the use of eddy current transducers such as employed by Chickering et al in U.S. Pat. No. 4,862,079 for monitoring wear and thickness in control rods of nuclear reactors.
It is known to use large electromagnets to induce magnetic fields in steel pipes and other articles; iron filings or other magnetic responsive particles are spread on the surface of the articles to reveal the patterns of the magnetic lines of force. (Hereafter, in this description, the term pipes may be used interchangeably with "steel pipes and other articles", as it is clear that magnetic-responsive articles other than steel pipes may be inspected in the same or a similar manner). Placement of the pipe in the magnetic field of an electromagnet causes the lines of magnetic force to pass through the pipe in predictable patterns which, however, are distorted by flaws; the distortions are made clearly visible in disruptions of the patterns of the magnetic particles. Commonly, the particles are colored or coated to reflect ultraviolet light.
To complete the saturation of the test piece with magnetic lines of force, it has been known to use both longitudinal and transverse magnetizing devices. That is, a longitudinal magnetic field is induced typically by electromagnetic coils around the pipe (but not touching it) and the transverse, or orthogonal, field is induced by devices in which the axis between the north and south poles is oriented to be more or less parallel to the pipe.
While solid cores work well in magnetic flux amplifiers, they create hazards such as possible shock and arc burns; they also tend to heat up, reducing their efficiency unless a cooling system is used. If no cooling system is used, the core may burn out prematurely.
Spierer, in U.S. Pat. No. 4,477,776, illustrates several different configurations of magnets around a test piece, including one in FIG. 4 in which the longitudinal field is "vectored" by placing the poles of the magnet in positions other than directly across the test piece. He uses both transverse and longitudinal fields, but does not use magnetic particles, relying instead on magnetic sensing means which generate output signals.
In U.S. Pat. No. 4,694,247, Meili et al rotate and advance the pipe on a bed of dry magnetic particles while the pipe is subjected to magnetization.
Jenks, in U.S. Pat. No. 4,931,731, like Spierer in the patent mentioned above, uses both longitudinal and transverse magnetic fields; unlike Spierer, he reads the flaws by observing disturbances in the resulting patterns of magnetic particles. He employs a particular circuitry for maintaining a predetermined balance between the two magnetic fields. See also Kamimura in Japanese application 56-31557. Both Jenks and Kamimura employ two coils to generate the longitudinal field.
Two coils are also used by Lam in U.S. Pat. No. 5,534,775 to generate the longitudinal field. Lam uses "time-varying" magnetic fields, and passes the current through the two coils in directions which tend to cancel the lines of force which do not contribute to the longitudinal field.
Many of the prior art constructions are bulky and difficult to use in the environment of a production plant. It is desirable to keep the inspection process simple while still thorough.